1. Technical Field of the Invention
This invention relates to an improved process for burning a fuel containing sulfur. More particularly, the invention relates to a process for burning a fuel containing sulfur in three stages to reduce the emission of particulates and sulfur compounds in the combustion gases.
2. Background Art
The combustion of fuels containing sulfur as well as incombustible ash-forming residues results in the need to control emission of particulates and sulfur compounds for environmental reasons. Since these sulfur compounds and particulates may constitute significant environmental hazards, much work has been devoted to the development of methods for preventing formation of these substances or cleansing them from the combustion gases.
With respect to the presence of sulfur in the fuel, it has been proposed to add materials to the fuel which will, at least at the combustion temperature, react with the sulfur to form sulfur compounds which may be removed, i.e. to prevent or mitigate the formation of sulfur oxide gases. Spurrier U.S. Pat. No. 1,007,153 proposed the addition of a salt, hydrate or oxide of one of the alkali metals as an additive to coke whereby the alkali would be carried into the pores of the coke where it may react with the sulfur upon heating to form sulfates and sulfides.
Trent U.S. Pat. No. 1,545,620 described saturating pulverized coke with water and co-mingling this with a mixture of pulverized limestone and hydrocarbon oil to form a plastic mass in which there is a close association between the sulfur and the limestone. When the mixture is coked, the limestone and sulfur react to form calcium sulfide.
McLaren et al U.S. Pat. No. 3,540,387 describes the addition of a carbonate, such as calcium carbonate, to a fluidized bed containing coal so that the sulfur is retained in the bed.
Robison et al U.S. Pat. No. 3,717,700 describes the use of a sulfur acceptor material in a first combustion zone to absorb the sulfur and then release it in a second zone to therefore concentrate most of the sulfur oxides in a small fraction of the flue gas.
Wall U.S. Pat. No. 4,102,277 describes incinerating sewage which has been dewatered with the aid of lime and then incinerated using high sulfur fuel. During incineration, the lime reacts with the sulfur in the fuel and with oxygen to form calcium sulfate for disposal and to prevent formation of polluting sulfur oxide gases.
It is also known to mix fuel with an additive to control or alter the melting or softening point of the ash or slag formed to facilitate removal thereof. Barba U.S. Pat. No. 1,167,471 discloses the addition of clay to powdered coal to raise the melting point of the ash to form a more satisfactory coating on metals being heat treated.
Benner et al U.S. Pat. No. 1,955,574 adds a reagent to coal to alter and/or control the melting or softening point of the slag to protect the furnace walls from molten slag. The softening point of coal ash is said to be raised by the addition of sand or a non-ferruginous clay or lowered by the addition of lime or soda. The melting or softening point is controlled by the patentee to permit the build-up of a thin layer of solid slag on the furnace walls to protect the refractory walls from molten slag which is formed in the interior of the furnace.
Romer et al U.S. Pat. No. 2,800,172 relates to the addition of a metal or a metal oxide, e.g. aluminum, magnesium or calcium, to a liquid fuel to alter the form of slag produced in a combustion chamber to an easily removed slag.
The controlling of the combustion temperature to insure the production of a molten slag to thereby reduce airborne particulates is also known. Jonakin U.S. Pat. No. 3,313,251 describes a method for processing coal slurries containing crushed coal and water wherein the temperature in the furnace is maintained above the melting point of the ash in the coal so that a molten residue is produced by the combustion process. The centrifugal action produced in a cyclone furnace causes this residue to impinge on the furnace walls where, under the influence of gravity, it flows to the bottom of the furnace where it may be removed.
It is also known to burn fuel in more than one stage to reduce smoke and sulfur oxide production by providing an air-fuel ratio in the first stage less than that for stoichiometric burning. Fraser et al U.S. Pat. No. 3,228,451 proposed burning fuel in such a two stage process wherein the fuel was burned in a first stage at an air-fuel ratio less than that for stoichiometric burning. The products of this combustion were then cooled and subsequently burned in a second stage with an excess of air which resulted in a lowering of the burning temperature.
Barsin U.S. Pat. No. 4,144,017 proposed burning fuel in several stages wherein the combustion air delivered to a primary furnace was regulated to introduce 50 to 70% of total stoichiometric air while maintaining the maximum combustion temperature at or below 2500.degree. F. to reduce the formation of nitric oxides. The combustion air delivered to the second stage or secondary furnace is also regulated to introduce 50 to 70% of total stoichiometric air to the second furnace while maintaining a combustion temperature at or below 2900.degree. F.
In Brown U.S. Pat. No. 4,232,615, assigned to the assignee of this invention, a process was disclosed for burning a pulverized carbonaceous material containing sulfur and ash wherein an additive was used capable of reacting during combustion with the sulfur in the material, and the fuel was burned in two stages where the first stage contained less than 100% of the theoretical air and was preferably at a temperature below 1100.degree. C. to thereby inhibit the formation of undesirable sulfur oxide gases and to assist in the removal of the sulfur as solid compounds. It was proposed therein that the first stage could be maintained at a temperature either below or above the melting point of the ash depending upon the desired conditions. It was further suggested that the additives used for reacting with the sulfur to form sulfur compounds might also have an effect upon the overall melting point of the ash either reducing or raising it, depending upon the particular compound used.
While all of the foregoing processes contributed to the reduction of the sulfur and/or particulates in the emissions from combustion processes, most of the processes either favor the removal of sulfur or the formation of an easily recoverable ash. For example, in the aforementioned Brown patent, if the slurry is burned in the first stage with less than 100% theoretical air at a temperature below the melting point of the ash, the sulfur removal is good both from the standpoint of the limitation of air aiding in the formation of thermally stable sulfide compounds rather than sulfites, and the reduced temperature preventing any sulfite compounds formed from decomposing to undesirable sulfur oxide gases. In addition, the reaction between the additives and sulfur is enhanced by the large surface area of the fine particulate particles. Furthermore, the reduced temperature reduces the formation of oxides of nitrogen as well.
However, the lower temperature, while being useful in more completely eliminating sulfur emissions, increases the problem with regard to particulates in the emissions since the combustion temperature is below the melting point of the ash and the ash, therefore, remains in a particulate form which is more difficult to remove from the gases.
On the other hand, if the first stage is carried out at a temperature above the melting point of the ash, any sulfite compounds formed may be more easily decomposed to the undesirable sulfur oxide emissions. Furthermore, the relatively small surface area of the molten slag on the burner wall slows down the reaction between additives and sulfur.
Thus, operation of the prior art processes represented a compromise at best wherein either the elimination of sulfur or the elimination of the particulates was preferred to the detriment of the other. It would, therefore, be highly desirable to provide a process wherein both sulfur and particulate removal was optimized to therefore reduce the emission of both of these undesirable material from the combustion process.